The present invention relates to control systems for hydrostatic transmission, and more particularly, to such systems which control the fluid pressure in the strokers utilized to control the swashplate position of a variable displacement pump.
Hydrostatic transmission and control systems therefore have become well known in the art. One of the common uses of such systems has been to transmit output torque from the engine of a vehicle into input torque to the vehicle drive wheels. This use of hydrostatic transmission has become especially common on large off-the-road vehicles such as agricultural equipment and, although the control system of the present invention may be advantageously applied to hydrostatic transmissions for a variety of uses, it is especially suited for use with those employed to drive agricultural vehicles, and will be described in connection therewith.
One of the problems associated with the use of hydrostatic transmission systems to drive large agricultural vehicles is that they have been subject to "creeping". This is a phenomenon which occurs when the control lever is placed in the neutral setting to de-swash the variable displacement pump and bring the vehicle to a stop, but, because the control valve does not completely shut off the flow of control pressure fluid to both of the strokers, the swashplate remains slightly displaced, resulting in continued fluid output from the pump and continued movement of the vehicle.
One of the favored solutions to the problem of creeping has been to utilize what is referred to as a "wide-band neutral" control spool in which the lands of the valve spool are positioned to block the flow of fluid from the control port to the stroker ports over a range of several degrees of selector position on either side of the neutral setting. The use of wide band neutral control spools has been largely successful in eliminating the problem of vehicle creeping.
Another problem which has arisen in connection with agricultural vehicles utilizing hydrostatic transmissions concerns the use of such vehicles on a sloping terrain where the vehicle alternately goes uphill, then downhill. These vehicles frequently include some type of accessory or agricultural implement, such as a combine, which operates at a given speed and, preferably, the vehicle should move at a nearly constant speed for efficient operation of the implement. However, as the vehicle goes uphill, the increased load on the pump tends to de-swash the pump slightly, thus slowing the vehicle. When the vehicle goes downhill, it tends to overrun the speed of the motor, thus driving the motor as a pump and generating pressurized flow back to the pump to operate the pump as a motor. When this occurs, the pump swashplate is biased toward a greater displacement, such that the stroker which was being actuated by control pressure must now be drained, and the stroker which was being drained must now be pressurized to return the swashplate to its original position. This reversal of stroker pressures requires that the control valve pass through the neutral band to shut off the stroker port which was previously open and open the stroker port which was previously blocked. This reversal of pressure in the strokers counteracts the effect of system pressure tending to increase the swash angle, thus maintaining the swashplate at some new position.
It will be apparent to those skilled in the art that this increase in the swashplate angle as the control valve passes through the neutral band will permit an undesirable increase in speed as the vehicle goes down a slope. It will also be apparent that the amount of undesirable downhill speed-up increases as the amount of neutral band increases. Normally, the above-described reversal would result in movement of the swashplate sufficient to move the control spool a total distance somewhat greater than the dimension of the neutral band. Depending upon the configuration of the follow-up linkage between the swashplate and the valve spool, the total swashplate movement permitted may be the same as the movement of the spool, or as much as several times greater.
Accordingly, it is an object of the present invention to provide a hydrostatic transmission control system which minimizes the amount of control spool travel and swashplate movement during the reversal of stroker pressures associated with the transition from uphill operation to downhill operation.
It is a further object of the present invention to provide a hydrostatic transmission control system which accomplishes the above-stated object without a detrimental effect on the ability of the system to maintain a zero swashplate displacement when the control is in the neutral setting.